Knowing how much water Tasmania has, and where, is fundamental knowledge for resource management and for understanding the implications of changes in water availability both spatially and over time. Changes in streamflow and water availability can have impacts on habitat condition and the quantity and quality of water available for human use. During periods of low flow, sediment may settle to the bottom of a stream and sections may become semi-stagnant, resulting in low dissolved oxygen concentrations. Algal growth will increase if there is adequate light, leading to algal blooms (some of these may be toxic). Salinity and water temperature may increase to values that alter the biota in the waterway.

Streamflows are often reduced by storing flood water in dams or artificial lakes and releasing it during periods of low river flow. This can come at the cost of reducing a river's capacity for flushing its accumulated sediment load.

The Water Assessment Branch of DPIPWE has more than 50 monitoring sites across the State. The majority of these sites have been established to monitor water quantity and water extraction and establish environmental flows.

The availability of stream flow data has improved since the 1997 and 2003 SoE Reports through the upgrading of the DPIPWE network of stream gauging sites. These data are also now more readily available to the community through the External linkWater Information System of Tasmania. These sites provide an extremely valuable long-term record for the State with monitoring for some sites dating back to 1923 (North Esk River), 1948 (Huon River), and 1956 (South Esk). Nineteen sites have been monitoring stream flow continuously since the 1960s and a further 22 sites have been monitoring stream flow since the 1970s or 1980s. The stream gauging network has improved in recent years with the inclusion of monitoring at sites including: Great Forester River (2003), Little Swanport (2004), Macquarie (2004) and Back Creek (2005).

Monitoring of streamflows higher in catchments in combination with the current DPIPWE network would provide a clearer indication of water availability and abstraction along a catchment. While the WIST system has been improved significantly, there are some limitations associated with data extraction from the website that constrain the analysis and presentation of data.

Monitoring salinity

The existing surface water monitoring network, although not developed for monitoring salinity alone, is a good source of information for determining salinity condition. A limitation is that many of the monitoring sites are located in less optimal places for monitoring salinity baseflow. All surface water monitoring sites are established to the Australian Standards set for station installation and water quality monitoring is undertaken under the Australian Standard AS/NZS 5667-6-1998 Water Quality – Sampling – Guidance on Sampling of Rivers and Streams (Internal linkBastick et al. 2007).

The TREND software developed by the CRC for Catchment Hydrology (Internal linkChiew et al. 2005) was used to test for trends and changes in DPIPWE streamflow data for those rivers with longer-term data available. TREND has 12 statistical tests, based on the WMO/UNESCO Expert Workshop on Trend/Change Detection and on the CRC for Catchment Hydrology publication Hydrological Recipes.

Of the 12 available tests, 8 showed a statistically significant decline in river flow in the South Esk River in the period 1957 to 2002. Step changes were identified for 1978 and 1986. Total yield for the South Esk River is about 750,000 ML of which about 50,000 ML is abstracted for other uses. It is likely therefore that increases in abstraction alone cannot account solely for the declining trend and that a statistically significant decline in flow caused by drought has occurred particularly over the last 15 years.

Examining the 35 year period from 1970 to 2004, CSIRO and Hydro Tasmania identified that the north coast and eastern half of Tasmania seem to be drying significantly at a rate of 4 to 8 mm/year, which is equivalent to a 140 to 280 mm decrease in rainfall over the 35 year period (Internal linkMcIntosh et al. 2006). Over the entire year, the rainfall decrease in the South Esk catchment is about 74 mm over the 35 years or about 8% of the annual total (Internal linkMcIntosh et al. 2006). Further information on these studies is available from the Hydro Tasmania website (External linkHigh resolution modelling of Tasmania's climate). Rainfall projections for the period 2006–2040 predict a future drying trend in the first half of the year in the South Esk catchment.

A stream flow record of 80 years is available for the DPIPWE North Esk River at Ballroom site (station number 76). This is an important record both because it is available over a long period and it is located at a site that is comparatively unaffected by agricultural water abstraction (although forestry activity in the upper catchment may have altered flows). Simillarly to the South Esk River, 8 of the 12 tests show a statistically significant decline in river flow in the North Esk River in the period 1927 to 2004 (data gaps prevented analysis up to present). Three different tests were is agreement that a step decline in the North Esk River occurred about 1975.

The Internal linktable shows the month and year when minimum flows were recorded from the DPIPWE network of sites for those sites with a record back to the 1970s. This table includes the most recent data available at the time of writing to June 2008. These data are also summarised in the following interactive map of minimum flows (requires External linkAdobe SVG Viewer).

The following plots show changes in discharge over time for two example sites. The Discharge from DPIW Stream Gauging Sites provides these plots (also known as Hydrographs) for the other sites from the DPIPWE stream gauging network.

Flow change is a condition variable referred to as RS_FLOW in the CFEV database. Flow change is a sub-index in CFEV that was derived with an expert rule system. The input variables were flow variability, abstraction and regulation. The draft CFEV technical report notes that riverine flow regimes are affected by urbanisation, land clearing, water abstraction and regulation by in-stream storages such as dams and weirs (Internal linkDPIW 2008). In many regulated river systems, there has been a reduction in the timing, frequency, magnitude and duration of minor and moderate flood events that are essential for sustaining riparian vegetation, river and floodplain ecosystems (Internal linkKingsford 2000).

The Internal linktable describes the flow change categories used in the CFEV flow change sub-index. The following map and the attached Internal linktable show the output of the CFEV flow change index at a State scale. For the purpose of a Statewide overview for this SoE Report, stream orders greater than or equal to three were selected only. Lower order (headwater streams) were not included in the analysis.

CFEV flow change category for river sections, based on flow change score

Within the structure of State of the Environment Report, an indicator can be related or associated with any number of issue reports (or vice versa). The data within an indicator is used to inform an issue report and any related recommendations. A summary of this indicator, including it's relevance to the particular issue, can be found within the 'Indicator' section of each of the linked issue reports below.